Thin-film, magnetic recording discs are widely used in the computer industry as a means for storing large amounts of digital data. Data are written onto and read off of a rapidly rotating recording disc by means of a magnetic head transducer assembly that flies closely over the disc. The most commonly used recording discs are of the type that include a magnetic oxide dispersion layer formed on a substrate or coated substrate. As an example, an iron oxide recording disc includes a dispersion of ferrimagnetic iron (III) oxide (.gamma.-Fe.sub.2 O.sub.3) micropowders in an epoxy resin binder. As is well known in the art, .gamma.-Fe.sub.2 O.sub.3 dispersions exhibit an acicular quality that imposes a limit upon the maximum digital storage density for iron oxide recording discs.
In order to provide magnetic recording discs with higher digital storage densities, magnetic alloy thin-film recording discs, which include a magnetic alloy formed on a substrate or coated substrate, have been developed. The magnetic medium is formed from ferrimagnetic alloys of high coercivity, such as magnetic cobalt alloys. As an example, one commercially available, magnetic alloy thin-film recording disc has a magnetic medium comprised of a micro-crystalline, ferrimagnetic cobalt-nickel alloy. A thin film of the cobalt-nickel is formed on a coated disc substrate comprised of an aluminum or aluminum alloy substrate coated with a non-ferrimagnetic nickel alloy.
Since the linear, digital recording density for any magnetic recording disc depends not only on the nature and thickness of the magnetic recording medium and the characteristics of the magnetic head, but also on the head-to-medium spacing, higher digital recording densities can be achieved by flying a magnetic head very closely (on the order of submicrons) over the surface of a recording disc. However, the extremely close operational spacing between a rotating recording disc and a magnetic head, as well as the start-stop operations of the magnetic recording system, result in an amount of head-disc dynamic contact that imposes severe wear-resisting requirements on the recording disc. The surface of the recording disc must exhibit a substantial degree of lubricity to insure a low coefficient of friction between the magnetic head and the surface of the recording disc, and thereby prevent the excessive wearing of the disc surface.
For recording discs of iron (III) oxide, chronium (IV) oxide, and other magnetic oxides, the necessary lubricating properties are typically provided by a lubricating polymer coating that binds to the epoxy of the magnetic oxide dispersion. However, in the case of a magnetic alloy thin-film recording disc, such as that noted above, no lubricating polymer has been found that adequately binds to the thin-film magnetic medium.
In order to provide the necessary lubricating boundary layer between the disc surface and the magnetic recording head, one approach has been to form a protective overlayer of rhodium over the magnetic medium. During the operation of the magnetic recording system, the rhodium overlayer reacts with organic vapors present in the environment, even in trace concentrations, to form amorphous organic deposits on the surface of the rhodium. The organic vapors are absorbed at the surface of the rhodium, where they undergo polymerization as a result of frictional activation during head-disc dynamic contact. This frictional activation process results in an accumulation of organic deposits that resemble mixed polymers in their properties, which are thus termed frictional polymers.
Initially, the frictional polymers deposited on the disc surface comprise thin, oily translucent films that provide remarkably effective lubrication. However, with the continued operation of the magnetic recording system and the resulting continued frictional activation, these frictional polymers further polymerize and become transformed into gummy layers. In this state, the frictional polymers cease to provide effective lubrication, resulting in the excessive wearing of the surface of the recording disc, the sticking of the magnetic head to the disc surface during static head-disc contact, and the aerodynamic instability of the magnetic head. The rate of buildup of the frictional polymers is difficult to predict, being determined by a number of factors such as the type and amount of organic vapors present in the environment, the rate of rotation of the disc, and the number of start-stop operations of the magnetic recording system.
The inability to control the buildup of non-lubricating organic deposits on the surface of the recording disc is a principal, if not the principal, disadvantage of rhodium-plated, magnetic recording discs. If, on the one hand, organic vapors are completely excluded from the disc environment, excessive wear of the disc surface occurs because of the loss of the lubrication provided by the frictional polymers. If, on the other hand, organic vapors are present, in however small concentrations, excessive buildup of the frictional polymer deposits inevitably occurs, particularly at the higher temperatures and higher rotational velocities associated with efficient disc storage operations, leading to the problems noted previously and eventually to the failure of the recording disc. At the present time, attempts to partially deactivate the rhodium overlayer in order to inhibit the formation of frictional polymers have been unsuccessful, as has been the search for a stable lubricating frictional polymer.
It is therefore a general object of the present invention to provide a wear-resistant, protective overcoating for a magnetic alloy thin-film recording disc and a method for forming the same. To this end, it is a specific object of the present invention to provide a protective overcoating that exhibits a high degree of lubricity and is effective adhered to the underlying magnetic medium.
Another object of the present invention is to provide such a protective overcoating that substantially does not catalyze the formation of frictional polymers on the surface thereof.